1. Field of the Invention
The present invention relates to a semiconductor device involving a heterojunction, and more particularly to a heterojunction field effect transistor operative in the micro wave and millimeter wave bands.
2. Description of the Related Art
A highly electron-mobile transistor and a dope channel heterojunction field effect transistor, both of which are heterojunction field effect transistors, are used in the micro wave and millimeter wave bands. A cross sectional structure of a highly electron-mobile transistor is shown in FIG. 7.
The structure of the highly electron-mobile transistor shown in FIG. 7 includes in the following order a semi-insulating substrate 104, a buffer layer 105, a channel layer 106, a first barrier layer 107, a second barrier layer 108 and a contact layer 109. Formed on the upper surface of the contact layer 109 are a source electrode 102 and a drain electrode 103. The source electrode 102 and the drain electrode 103 are ohmic-joined with the contact layer 109. Further, a gate electrode 101 is formed on the second barrier layer 108 after selectively recess-etching the contact layer 109. Moreover, the highly electron-mobile transistor is protected by a protection film 111. Here, a recess portion formed between the gate electrode 101 and the source electrode 102, as well as a recess portion formed between the gate electrode 101 and the drain electrode 103, are each referred to as eye-empty areas (eye-empty areas 110 shown in FIG. 7) which serve as important portions providing a significance effect on the voltage durability of the heterojunction field effect transistor. Of course, the eye-empty areas 110 are formed by part of the contact layer 109.
The recess-etching processed shape of the heterojunction field effect transistor not only has a one-step recess structure shown in FIG. 7, but also have a two-step recess structure including two steps. FIG. 8 shows as an example a cross sectional structure of a highly electron-mobile transistor having a two-step recess structure.
However, with regard to the one-step recess structure shown as a conventional structure in FIG. 7, since the contact layer 109 forms an ohmic-junction with the source electrode 102 and the drain electrode 103, it is formed by an n-type GaAs layer having a high dopant concentration. Further, as described above, since the eye-empty areas 110 are formed by the contact layer 109, the eye-empty areas 110 also have a high dopant concentration, exhibiting a high carrier concentration. For this reason, when an electric field is applied to an area formed between the gate electrode 101 and the source electrode 102 as well as to an area formed between the gate electrode 101 and the drain electrode 103, centralized in the eye-empty areas 110, the carrier concentration of the eye-empty areas 110 is relatively high, forming a weak insulating strength, hence causing a breakdown in a low electric field.
As a measure for inhibiting a breakdown in a low electric field, there has been in use the aforesaid two-step recess structure shown in FIG. 8. As shown in FIG. 8, the transistor having the two-step recess structure comprises in the following order a semi-insulating substrate 124, a buffer layer 125, a channel layer 126, a first barrier layer 127, a second barrier layer 128, and a contact layer 130. Further, a connection layer 129 consisting of an n-type GaAs having a low dopant concentration is formed between the contact layer 130 and the second barrier layer 128. In this structure, a path extending from the gate electrode 121 to the drain electrode 123 as well as other paths extending from the source electrode 122 and the gate electrode 121 to the drain electrode 123, are all formed into a two-step structure. As a result, an applied electric field can be dispersed into several portions based on the plurality of steps, obtaining an effect that an electric field applied to each step of a multi-step structure is smaller than that applied to a one-step recess structure, thereby enabling the transistor to have an improved voltage durability.
In addition, since the connection layer 129 is formed by an n-type GaAs layer having a low dopant concentration, an insulating capability of the eye-empty areas 132 is increased, so that the transistor has an improved voltage durability.
However, with the above-described arrangement, the connection layer 129 formed by an n-type GaAs layer having a low dopant concentration has a low carrier concentration and thus has an undesirably increased resistance. Moreover, since a surface depletion layer that occurs on the surface of the eye-empty areas 132 become large, the high resistance of the eye-empty areas is further enhanced. For the reasons described above, since an electric current path extending from the drain electrode to the source electrode becomes narrow, there is a problem that a series resistance of the heterojunction field effect transistor is increased. Moreover, since the surface depletion layers that occur on the surfaces of the eye-empty areas 132 are formed uniformly, once an electric field is applied to an area formed between the gate electrode 121 and the source electrode 122, as well as to an area formed between the gate electrode 121 and the drain electrode 123, the electric field will be centralized on some corner portions, such as the gate electrode end and the recess processing end portions in the eye-empty areas 132 (on which surface depletion layers have already occurred). Hence it causes the heterojunction field effect transistor to have a deteriorated voltage durability.
In particular, an increased series resistance of the heterojunction field effect transistor as well as a decreased voltage durability thereof are the most significant factors responsible for some deteriorated characteristics of an oscillator and a power amplifier which are required to have a high gain, a high output and a high efficiency and operative from the micro wave band to the millimeter wave.